A mobile terminal, also known as a User Equipment (UE), wireless terminal and/or mobile station is enabled to communicate wirelessly in a wireless communication network, some-times also referred to as a cellular radio system. The communication may be made, e.g., between two mobile terminals, between a mobile terminal and a wire-connected telephone and/or between a mobile terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks. The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc.
The mobile terminal may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The mobile terminal in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile terminals, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile terminal, a stationary entity or a server.
The wireless communication network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node or base station, e.g., a Radio Base Station (RBS) or Base Transceiver Station (BTS), which in some networks may be referred to as “eNB,” “eNodeB,” “NodeB” or “B node,” depending on the technology and/or terminology used.
Sometimes, the expression “cell” may be used for denoting the radio network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base station site. One radio net-work node, situated on the base station site, may serve one or several cells. The radio net-work nodes may communicate over the air interface operating on radio frequencies with any mobile station within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks. LTE is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using a different radio interface together with core network improvements.
LTE-Advanced, i.e., LTE Release10 and later releases are set to provide higher bitrates in a cost efficient way and, at the same time, completely fulfil the requirements set by International Telecommunication Union (ITU) for the International Mobile Telecommunications (IMT)-Advanced, sometimes also referred to as 4G (abbreviation for “fourth generation”).
Communications in emergency situations cannot rely only on cellular systems infrastructure, such as for example the LTE system, as it can be out of function, like in cases of earthquakes, tsunamis, snow storms, hurricanes, etc. In some areas, there may not even exist any cellular system coverage at all. Therefore there is currently ongoing standardization work to specify technical solutions that would allow LTE terminals, or mobile terminals to directly communicate with each other and even possibly relay information sent from a single terminal via another terminal, or multiple other terminals. The direct communication between terminals, also known as Device-to-Device (D2D) communication, should be possible both with and/or without the presence of LTE cellular infrastructure. In other words, such private handheld devices are supposed to form a backup, ad hoc communication network in disasters when existing communication infrastructure is not functioning or there is no cellular coverage to begin with. For public safety applications, broadcast communications may be utilized, i.e., the same information could be received by a number of D2D users.
A further application is that a mobile terminal in vicinity of other mobile or stationary terminals should be able to discover such terminals and thereafter be able to establish D2D communication. Discovery mechanism may also be applicable to commercial applications where a D2D user could set up direct communications with close-by friends or be used for advertising. Thus, such discovery may occur even if the terminals are under the coverage of an LTE system, i.e., independently of the LTE system.
In order to establish the initial contact, each mobile terminal should be able to transmit and receive D2D Synchronization Signals (D2DSS), which could serve at the receiver, e.g., both as the discovery signals and as a tool to establish time and frequency synchronization with the transmitting mobile terminal. The basic signal property of a D2DSS is that it should provide an impulse-like aperiodic auto-correlation function in order to provide reliable detection at the receiver. It should also be possible to detect the D2DSS with low-complexity with a corresponding matched filter in the receiver. As there might be a number of concurrent D2D communication links in a rather small geographical area, multiple D2DSSs with low cross-correlation should be available, which may be selected, e.g., randomly, based on signal measurements, or by any predefined rules, by mobile terminals so that even in case of signal collisions at the receiver, there is a chance that a D2DSS can be reliably detected.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the mobile terminal. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the mobile terminal to the radio network node.
In one example, D2D communications may be specified in uplink (UL) resources, i.e., in the UL carrier for Frequency-Division Duplexing (FDD) or in UL subframes for Time-Division Duplexing (TDD). In the latter case, a mobile terminal which is connected or is connecting to the LTE network may receive a D2DSS sent from another mobile terminal which is not synchronized to, or located within the coverage of the LTE network, while receiving synchronization signals sent in the downlink (DL) from the radio network node. The D2DSS should therefore be clearly distinguishable from all the LTE DL synchronization signals.
In order to minimize the complexity of the LTE terminals that support D2D communication, it is clear that the basic D2D transmission method should be the same as either on LTE DL, which is Orthogonal Frequency Division Multiplexing (OFDM), or as on LTE UL, which is Single-Carrier Frequency Division Multiple Access (SC-FDMA). It can be assumed that the future LTE D2D communications and the D2DSS can operate either in LTE FDD UL bands or in UL subframes in TDD mode. SC-FDMA and OFDM are technically both OFDM signals; however, SC-FDMA uses a ½ subcarrier shift and allows modulation of the all sub-carriers, in contrast to OFDM which uses an un-modulated DC subcarrier, wherein the frequency would be equal to the Radio Frequency center frequency of the radio network node.
There were two key aspects that governed the design of the Primary Synchronization Signal (PSS)/Secondary Synchronization Signal (SSS) in Rel-8, which largely also should be prioritized for a D2DSS: detection performance and receiver complexity.
Detection performance depends on the amount of resources allocated to the synchronization signal, as well as the characteristics of the signals, e.g., cross-correlations.
The receiver typically performs matched filtering to detect the PSS. Receiver complexity depends on the ability to utilize certain signal properties to mainly reduce the number of complex-valued multiplications in the receiver. The PSS has a time-domain central symmetry, i.e., a signal value appears up to two times in an OFDM symbol which allows for reducing the number of multiplications by ˜50% by performing addition of symmetric samples prior to multiplication with the replica sample. There are three different PSSs, which are obtained from three different modulation sequences (i.e., PSS sequences). Further-more, two of the PSS sequences constitute a complex-conjugated sequence pair and due to the time-domain central symmetry, they also become a complex conjugated signal pair, which makes it possible to detect both PSSs with the same multiplication complexity as detecting one PSS. The SSS is based on m-sequences, for which Fast Hadamard Trans-forms could be used in the detector. It has been observed that the cell searcher contributes to 10-15% of the total baseband cost of the LTE modem. Therefore, it is crucial that a D2DSS will support low-complex receiver implementations and that as much as possible from the existing PSS/SSS detector implementations may be reused.
The D2DSS transmitted in the LTE TDD mode and the existing PSS transmitted from a radio network node, or eNodeB, may give rise to mutual interference. For example, as illustrated in FIG. 1a, a legacy mobile terminal may try to detect the PSS of the radio network node of the LTE system, e.g., for cell selection, while not being able to succeed in accessing a cell if D2D terminals in its vicinity transmit a D2DSS having large cross-correlation with the PSS, see FIG. 1a. In this situation the legacy terminal has no prior synchronization to the radio network node and will search for the PSS also in UL subframes, wherein the D2DSS may be transmitted.
In another illustrated example in FIG. 1b, a D2D terminal located outside LTE network coverage will not be able to access any cell but may occasionally still receive the LTE synchronization signals, PSS/SSS, see FIG. 1b. These signals would constitute interference while trying to detect the D2DSS. In this situation the D2D terminal has no synchronization to the radio network node and the PSS may be detected in subframes, wherein the D2DSS may be received.
Even if the LTE PSS and the D2DSS would use different waveforms, it can be shown that the cross-correlation between the LTE PSS (based on OFDM waveforms) and a D2DSS obtained from SC-FDMA waveform modulated with the same PSS sequence, exhibits two strong cross-correlation peaks, corresponding to about 50% of the signal energy. Further-more, the peak cross-correlation is more than 50% higher than the maximum auto-correlation side lobe of D2DSS. Table 1 gives an example of correlation values when the D2DSS is using the same modulation sequence as for the PSS but is utilizing the SC-FDMA waveform. The term root index refers to a parameter in the modulation sequence definition and different root indices result in different sequences.
TABLE 1Cross-correlationRootAuto-correlation:Cross-correlation:with PSS:indexMaximum falseMaximum falseMaximum false peak,(u)peak correlationpeak, (root index)(PSS root index)u = 250.240.38, (u = 34)0.47, (u = 25)u = 290.240.40, (u = 34)0.47, (u = 29)u = 340.240.38, (u = 25)0.47, (u = 34)
Such interference levels are not desirable, as the maximum cross-correlation should not be significantly higher than the maximum auto-correlation side lobe, in order to be able to keep the same detection threshold in the receiver as if there were no interference, i.e., as on an Additive White Gaussian Noise (AWGN) channel. With these cross-correlation peaks, the detection threshold has to be increased in order to preserve the targeted false alarm rate, which will cause the detection probability to be decreased.
The LTE PSS sequence is chosen and mapped to the subcarriers in such a way that the resulting PSS is centrally symmetric in the time-domain. The PSS is generated as an OFDM signal, where the DC subcarrier, i.e., frequency k=0 is un-modulated. A discrete form of the signal may be represented by:
      s    ⁡          [      n      ]        =            1              N              ⁢                  ∑                  k          =          0                          N          -          1                    ⁢                          ⁢                        H          ⁡                      [            k            ]                          ⁢                  e                                    j              ⁢                                                          ⁢              2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              nk                        N                              
for n=0, 1, . . . , N−1. In order to obtain central symmetry in the time-domain, i.e., s[n]=s[N−n], n=1, . . . , N−1, the PSS is mapped to the subcarriers such that the following relation holds for the Fourier coefficients H[k]=H[N−k], k=1,2, . . . , N−1.
The central symmetry of N−2 samples of the PSS may be used to reduce the number of multiplications in the corresponding matched filter. For example, if the PSS has length of N samples, it can be shown that there are N−2 centrally symmetric samples in the PSS signal, i.e., there are (N−2)/2 unique sample values and additionally 2 samples which may not be equal to any other sample. Thus, by performing addition of the symmetric samples prior to multiplication with the replica symbol, the matched filter can be implemented by (N−2)/2+2 multiplications per single correlation, which is reduction of about 50% compared to the direct implementation which requires N multiplications, i.e., one multiplication per input sample. An example of an LTE receiver for the PSS is illustrated in FIG. 1c, where “*” denotes complex conjugation and su[n] are the values of the PSS with root index u.
FIG. 1c thus illustrates an efficient matched filter for PSS signal detection using N samples. Furthermore, three different PSSs are defined in LTE, which are obtained from three different PSS sequences. Two of the sequences constitute a complex conjugated version of each other. That is, there are two root indices u and v which generates PSS sequences such that the resulting PSSs are related by su[n]=sv*[n]. Therefore, since a complex conjugate only implies sign change to the imaginary part of the received sample, it is possible to detect both these PSSs with the multiplication complexity of just one of the sequences. That is, no extra multiplications are needed for computing the correlation of the conjugated signal, i.e., a complexity reduction of 50%. The central symmetry is preserved for any value of root-index u, allowing having just a single matched filter structure, with fixed connections between hardware elements, which can be thus reused for the detection of different D2DSS signals by changing just the replica signal.
The centrally symmetric PSS is obtained from the LTE PSS sequence d[n] generated from a frequency-domain Zadoff-Chu sequence of length 63 according to:
            d      u        ⁡          [      n      ]        =      {                                                      e                                                -                  j                                ⁢                                                      π                    ⁢                                                                                  ⁢                                          un                      ⁡                                              (                                                  n                          +                          1                                                )                                                                              63                                                      ,                                                              n              =              0                        ,            1            ,            …            ⁢                                                  ,            30                                                                          e                                                -                  j                                ⁢                                                      π                    ⁢                                                                                  ⁢                                          u                      ⁡                                              (                                                  n                          +                          1                                                )                                                              ⁢                                          (                                              n                        +                        2                                            )                                                        63                                                      ,                                                              n              =              31                        ,            32            ,            …            ⁢                                                  ,            61                              
where u∈{25,29,34} is referred to as a set of root indices. The sequence d[n] shall be mapped to the resource elements according to:
                    a                  k          ,          l                    =              d        ⁡                  [          n          ]                      ,          n      =      0        ,    1    ,    …    ⁢                  ,    61        k    =          n      -      31      +                                                  N              RB              DL                        ⁢                          N              sc              RB                                2                .            
The time-continuous low-pass signal sl(p)(t) on antenna port p in OFDM symbol l in a downlink slot is defined by:
                    s        l                  (          p          )                    ⁡              (        t        )              =                            ∑                      k            =                          -                              ⌊                                                      N                    RB                    DL                                    ⁢                                                            N                      sc                      RB                                        /                    2                                                  ⌋                                                          -            1                          ⁢                                  ⁢                              a                                          k                                  (                  -                  )                                            ,              l                                      (              p              )                                ·                      e                          j              ⁢                                                          ⁢              2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              k              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                              f                ⁡                                  (                                      t                    -                                                                  N                                                  CP                          ,                          l                                                                    ⁢                                              T                        s                                                                              )                                                                        +                        ∑                      k            =            1                                ⌈                                          N                RB                DL                            ⁢                                                N                  sc                  RB                                /                2                                      ⌉                          ⁢                                  ⁢                              a                                          k                                  (                  +                  )                                            ,              l                                      (              p              )                                ·                      e                          j              ⁢                                                          ⁢              2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              k              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                              f                ⁡                                  (                                      t                    -                                                                  N                                                  CP                          ,                          l                                                                    ⁢                                              T                        s                                                                              )                                                                                            ⁢    for                      ⁢          0      ≤      t      <                        (                                    N                              CP                ,                l                                      +            N                    )                ×                  T          s                                        ⁢    where                      ⁢                  k                  (          -          )                    =                        k          +                                    ⌊                                                                    N                    RB                    DL                                    ⁢                                      N                    sc                    RB                                                  2                            ⌋                        ⁢                                                  ⁢            and            ⁢                                                  ⁢                          k                              (                +                )                                                    =                  k          +                      ⌊                                                            N                  RB                  DL                                ⁢                                  N                  sc                  RB                                            2                        ⌋                    -          1                    and ak,l(p) is the content of resource element (k, l) on antenna port p. The variable N equals 2048 for Δf=15 kHz subcarrier spacing and 4096 for Δf=7.5 kHz subcarrier spacing. The entities NCP,l, NRBDL and NscRB are further defined in the LTE specifications.
The SC-FDMA waveform is in LTE such that the time-continuous low-pass signal sl(p)(t) for antenna port p in SC-FDMA symbol l in an uplink slot is defined by:
            s      l              (        p        )              ⁡          (      t      )        =            ∑              k        =                  -                      ⌊                                                            N                  RB                  UL                                ⁢                                  N                  sc                  RB                                            2                        ⌋                                                ⌈                                                    N                RB                UL                            ⁢                              N                sc                RB                                      2                    ⌉                -        1              ⁢                  a                              k                          (              -              )                                ,          l                          (          p          )                    ·              e                  j          ⁢                                          ⁢          2          ⁢                                          ⁢                      π            ⁡                          (                              k                +                                  1                  /                  2                                            )                                ⁢          Δ          ⁢                                          ⁢                      f            ⁡                          (                              t                -                                                      N                                          CP                      ,                      l                                                        ⁢                                      T                    s                                                              )                                          
for 0≤t<(NCP,l+N)×Ts where k(−)=k+└NRBULNscRB/2┘. The variable N equals 2048 for Δf=15 kHz subcarrier spacing and ak,l(p) is the content of resource element (k, l) on antenna port p. The entities Ts, NCP,l, NRBUL and NscRB are further defined in the LTE specifications.
In the context of this disclosure, an SC-FDMA waveform is referring to multi-carrier signal without any un-modulated DC subcarrier and where the subcarriers are allocated with half a subcarrier offset in relation to the DC frequency.